The investigations are designed to isolate and examine individual central neurons responsive to noxious stimuli, evaluate the effects of centrally acting drugs on their responsiveness, and examine the neuroanatomy and neurophysiologic mechanisms of focal brain stimulation-induced analgesia. Glass micropipettes will be employed for purposes of extracellular unit recording in the cat brains stem reticular core and thalamic intralaminar nuceli. Noxious stimuli will include electrical tooth pulp stimulation, electrical stimulation of the superficial radial and caudalcutaneous sural nerves, and radiant heat. The effects of morphine and other centrally acting drugs and saline on unitary responses to these noxious stimuli will be compared to their effects on unitary responses to somatic non-noxious stimuli for purposes of determining the significance of drug effects. Additional experiments will examine the neurophysiology and neuroanatomy of focal brain stimulation-induced analgesia. The objective of these investigations is to elaborate on the electrophysiologic mechanisms by which analgesic drugs exert their effects to obtund pain.